An example is in multimodal polymerisation reactions, where the polymers are typically manufactured in reactors connected in series with reaction conditions being differentiated in each reactor. In order to have maximum control of final product properties, it is preferred to have full and independent control of the molecular weight and the density of the polymer produced in each reactor; molecular weight is typically controlled using hydrogen. Consequently it is usually necessary to remove hydrogen from the product stream of a first reactor operating at higher hydrogen concentration than a second reactor in series with the first. The polymer from an upstream reactor to be fed to a downstream reactor in series is typically withdrawn with diluents (gaseous and/or liquid), catalysts and reagents such as monomer(s), comonomer(s), molecular weight control agents such as hydrogen, and cocatalysts. Various technical solutions are known to remove these undesired diluents and/or reagents, including hydrogen, either fully or partially from the polymer prior to its entry into the downstream reactor(s). Such techniques typically include pressure reduction to vaporise undesired components.
In EP 603935A a process is described in which a bimodal polyethylene is produced in series reactors with a low molecular weight homopolymer component being formed in the first reactor and a high molecular weight copolymer component being incorporated in the second reactor, with hydrogen being used to control the molecular weight. There is no discussion of how to remove residual hydrogen between the reactors. In EP 192427A and EP 897934A a significant pressure reduction between the two reactors is employed to remove at least a portion of the hydrogen present. This process is acceptable when the diluent remains substantially in the liquid phase under the pressure reduction conditions required to achieve the desired removal of hydrogen: however if a more volatile diluent is employed, or if a greater degree of hydrogen separation is required, then a more effective method is desirable. Slurry processes employing light (ie relatively volatile) solvents exhibit certain advantages over heavier solvent systems. For example, polyolefin oligomers tend to be less soluble, and the solvent is readily and substantially completely removed from the polymer product. However, hydrogen gas must be virtually completely removed between prior and subsequent stages, otherwise process control of the subsequent stage is difficult and high molecular weight may be impossible to attain. Light solvents tend to flash away with the hydrogen. If too much solvent flashes off, solids in the slurry take-off increase to such a high level that the slurry may no longer be pumpable. If solvent flash-off is reduced, hydrogen separation is poor. A further difficulty is that polymer entrained in the flash gas is still catalytically active and may polymerise further, causing problems with fouling of any apparatus employed for hydrogen removal or other separation. Thus it is necessary either to remove or to deactivate the residual polymer. Hence it can be seen that for this type of polymerisation reaction there is a need for an improved process for removing hydrogen between stages.
In US 2003/0191251 two flash vessels are used for the separation of hydrogen from a light diluent between polymerisation reactors. Each vessel has only one equilibrium stage. Significant diluent make-up is required after the first flash step due to a high diluent loss.
In U.S. Pat. No. 3,658,780 a polypropylene slurry withdrawn from a polymerisation reactor is treated with catalyst removal agents and the catalyst then washed out, thereby rendering the stream catalytically inactive, prior to fractionation of the stream to remove hydrogen.
In U.S. Pat. No. 6,045,661 a stream withdrawn from a reactor polymerising ethylene and hexene in isobutane is passed through flash vessels, and entrained polymer particles removed in a cyclone. At least a portion of the vapour is then compressed before being passed to a fractionator to separate the components. In this process it is stated that removal of entrained solids ensures that the fractionated material is not catalytically active.